Ernst has emerged as a high-profile candidate in recent months. She's considered a potential threat against the all-but-certain Democratic nominee, Rep. Bruce Braley, if she wins her party's nomination on June 3.

Republicans view Iowa as a potential pick-up seat in their quest to retake the Senate.

"Joni’s proven record in the state Senate, her decades of leadership as a military officer, and her commitment to the values that were passed on to her as a girl on a Montgomery County farm make it clear to me that she is the very best Iowa has to offer," Romney said in a statement distributed by Ernst’s campaign.